One- and two-dimensional electron, ion or photon impact position detectors are used in a variety of applications, including position sensitive atom probes, spectrometry, electron spectroscopy, atomic collision process observations, astronomical detectors such as those used for single photon imaging in very low light for ground and space based telescopes, low light night vision devices, and medical imaging equipment. Several designs have been developed for detectors capable of determining the point of occurence in two dimensions of an event such as the impact of a charged particle or photon at a discrete position on the detector.
One type of detector which is particularly useful for detecting charged particles, but which also may be utilized to detect photon impact, combines a microchannel plate with a detector anode which has electrodes arranged in planar fashion on a flat substrate. In what is commonly referred to as a wedge-and-strip anode, three electrodes are formed on the substrate: one electrode comprises a series of rectangular strips which increase progressively in width from one side of the anode to the other, another electrode comprises a series of identical wedges which extend across the anode between the strips, and the third electrode spans the interstices between the other two electrodes. An example of such a wedge-and-strip anode is shown in U.S. Pat. No. 5,061,850, where the detector is used in a position sensitive atom probe. Various designs for wedge-and-strip detectors are also shown in U.S. Pat. Nos. 3,209,201 to Anger, 4,395,636 to Anger, et al., 4,870,265 to Asmussen, et al., and in articles by C. Martin, et al., "Wedge-and-Strip Anodes For Centroid-Finding Position Sensitive Photon and Particle Detectors," Rev. Sci. Instrum. Vol. 52, No. 7, Jul. 1991, pp. 1067-1074, and C. Martin, et al., "Mosaic Wedge-and-Strip Arrays For Large Format Microchannel Plate Detectors," IEEE Transactions on Nuclear Science, Vol. 36, No. 1, February 1989, pp. 836-840. By amplifying the charge which results from a single event, such as a particle or photon impact, allowing the amplified charge cloud to land on the three (or sometimes four) electrodes, and integrating the charge on each electrode, the position of the event in two dimensions can be calculated. In a typical wedge-and-strip anode design having three electrodes A, B and C, where the measured charges on the electrodes from a single event are Q.sub.a, Q.sub.b and Q.sub.c, the X and Y position of impact of the incident particle or photon can be calculated from the formulas: EQU X=2Q.sub.a /(Q.sub.a +Q.sub.b +Q.sub.c) EQU Y=2Q.sub.b /(Q.sub.a +Q.sub.b +Q.sub.c)
Wedge-and-strip anodes are widely used because they are relatively easy to fabricate, exhibit little geometric distortion, and have high spatial resolution. For microchannel plate outputs in the range of 10.sup.6 to 10.sup.7 electrons, amplifier noise is less than 10.sup.3 electrons and images of 10.sup.5 to 10.sup.7 pixels can be obtained.
A main shortcoming of wedge-and-strip anode detectors is that it is not possible to detect the occurrence of multiple simultaneous events using such detectors. If two events occur simultaneously on a single wedge-and-strip anode, measurement of the charges on the electrodes will provide erroneous information, typically indicating that a single event has occurred at a position between the location of the two real events. When such simultaneous events do occur, there is no information from the wedge-and-strip anode which indicates the arrival of simultaneous events.